As is well known in the ion exchange arts, metal ions adsorbed on cation exchange resins can be eluted with aqueous solutions of strong mineral acids, such as hydrocholoric, nitric, and sulfuric acids. The adsorbed metal ions are exchanged for hydrogen ions so that the resin is returned to its hydrogen form for further use in the removal of metal ions from solutions. Commonly, such metal ion elution and cation exchange resin regeneration is carried out with relatively dilute acid solutions, and these are prepared by adding water to commercial concentrations of the regenerating acid. Since concentrated mineral acids produce heat when diluted with water, cooling of the regenerating acid solutions may be required with energy being used for the regeneration. Further, the use of dilute acid solutions results in a large volume of waste solution which presents a disposal problem.
The foregoing problems were encountered in the development of a system for removing principally magnesium and other cations from phosphoric acid, especially phosphoric acid prepared from phosphate rock having relatively high concentrations of magnesium, as is common with lower grades of phosphate rock. The system utilized moving folded beds of cation exchange resin for the adsorption and elution of the magnesium. A system of this kind is described in U.S. Pat. No. 4,280,904. The resin regeneration as described in this patent, is carried out with various acids including sulfuric acid. In experiments conducted prior to the present invention, the elution and regeneration was carried out with 3N sulfuric acid (approximately 17% H.sub.2 SO.sub.4 concentration). This eluting solution was prepared by adding water to commercial grade sulfuric acid (viz. 93%). The spent eluting solution which contained the removed magnesium together with eluted calcium gave processing and disposal problems. It was difficult and expensive to recover the magnesium as a precipitated solid, and yet it was undesirable to dispose of the large volume of spent eluting solution without further processing. These problems are overcome by the present invention.
Wet process phosphoric acid prepared from phosphate rock will contain organic matter which may adhere to the resin beads as transferred for regneration. It is therefore another important advantage of the method of this invention that such organic matter does not cause any operational problems even though the resin beads are repeatedly regenerated and recycled to the phosphoric acid treating column. The high concentration of H.sub.2 SO.sub.4 in the regenerating column The high concentration of H.sub.2 SO.sub.4 in the regenerating column facilitates its removal from the resin beads by the regenerating solution.